1. Field of the Invention
The present invention relates to a magnetic recording head capable of recording information on a magnetic recording medium by applying a high-frequency magnetic field to the medium to drive magnetic resonance and inducing magnetization switching of the recording medium, and a magnetic recording apparatus incorporating the magnetic recording head.
2. Description of the Related Art
With recent improvement in performance of computers, and speed and capacity of networks, amounts of information distributed in the form of digital data have been dramatically increasing. In this connection, a storage device capable of inputting and outputting large amounts of information at a high speed is needed to efficiently receive/transmit and extract such large amounts of information. With an increase in the high areal recording density, magnetic disks have become more prominently susceptible to a problem that signals which are once recorded gradually decrease due to thermal fluctuation. Since a magnetic recording medium is an aggregate of magnetic microcrystals, a decrease in the volume of each of the microcrystals causes the above problem. It is believed that often-used thermal fluctuation indicators Kβ (=KuV/kT; Ku: magnetic anisotropy, V: particle volume, T: absolute temperature, and k: Boltzmann constant) need to be 70 or higher to achieve adequate thermal fluctuation resistance stability. Given that Ku and T (material, environment) are fixed, the smaller V a microcrystal has, the more likely the magnetization is switched due to the thermal fluctuation. As the volume of a recording film occupied by 1 bit decreases with an increase in the high areal recording density, V should be reduced and thus the thermal fluctuation cannot be ignored. If Ku is increased to control the thermal fluctuation, a switching field needed for magnetic recording will exceed a writemagnetic field that can be generated by a recording head, which will result in disabled recording.
In order to avoid the problem, Zhu and others of CMU disclose microwave assisted magnetic recording (MAMR) technology in Patent Document 1. As shown in FIG. 1A, MAMR is recording in a way that a microwave field from a spin torque oscillator (STO) arranged adjacent to a main pole, in addition to a write field from the main pole of a perpendicular recording head, is applied to a magnetic recording medium 7 with high magnetic anisotropy and turns a recording target area into a magnetic resonance state to reduce the switching field. This enables recording onto a microwave irradiation area of a magnetic recording medium capable of high areal recording density exceeding 1 Tbit/in2 on which a conventional recording head has difficulty in recording due to an insufficient writemagnetic field. The STO generates microwaves (high-frequency magnetic fields) by conveying spin torques from a reference layer 31 to an adjacent field generation layer (FGL) 32 via Cu, and rapidly rotating magnetization of the FGL 32, which is an in-plane free layer, in a plane.
As MAMR utilizes the magnetic resonance phenomenon, an effective microwave field component is a polarized field component in anti-clockwise direction, which is a same rotating direction as precession of the recording medium magnetization. On the other hand, as shown in FIG. 1B, microwave fields from the FGL 32 which is a microwave field source of STO is an ellipsoidally polarized field whose rotating directions are dependent on a magnetization rotating direction of the FGL, and rotate in inverse directions before and behind the FGL 32, when viewed in a direction of head running. Thus, an anti-clockwise polarized field which is effective for MAMR is created only on one of the sides before and behind the FGL 32. Thus, a rotating direction of magnetization of the FGL 32 needs to be switched every time polarity of a main pole is switched. A realistic method for such switching is to switch magnetization of a reference layer, which acts as a spin torque source, according to a main pole field Hext while keeping STO drive currents constant as disclosed in Patent Document 2 and Patent Document 3 (See FIG. 2A and FIG. 2B).
In this case, since it is considered that spin torques necessary for driving the FGL cannot be obtained while magnetization of the reference layer is switching, high-speed magnetization switching of the reference layer is needed. Patent Document 2 discloses a technology for reducing magnetic coercive force of a reference layer of the STO disclosed in Patent Document 1 and switching reference layer magnetization by a main pole field, and a technology for increasing a switching speed by placing a magnetic body with high magnetic flux density adjacent to a reference layer. Patent Document 2 also discloses a technology for substantially making a part of a main pole or an auxiliary magnetic pole serve as a reference layer. A main pole is provided with a lip portion, a high-frequency magnetic field generator is arranged with a spin scattering layer interposed in between, and an electric current is supplied so that spin torques act in a direction to suppress an influence of a magnetic field of the main pole on the FGL. This configuration allows a magnetic field flowing into the radio-frequency generator from the main pole to enter perpendicular to a film surface. As the main pole is used as a spin source, a high-frequency magnetic field generator driving current which can achieve generation of a maximum high-frequency magnetic field independent of polarity of the main pole can be set according to a desired frequency.
In addition, Patent Document 4 and Patent Document 5 disclose a technology in which a pair of equivalent FGLs are configured to rapidly rotate their magnetization while keeping the magnetization antiparallel to each other, and a one-way high-frequency magnetic field which is generated from end faces of the FGLs and parallel to a recording medium face is applied to a medium for efficiently switching medium magnetization irrespective of the FGLs rotating direction.    [Patent Document 1] US 2008/0019040 A1    [Patent Document 2] JP 2009-070541 A    [Patent Document 3] WO 2009/133786 A1    [Patent Document 4] JP 2008-277586 A    [Patent Document 5] JP 2008-305486A